1. Field of the Invention
The present invention relates to liquid crystal displaying techniques, and in particular to a method for selecting FRC (Frame Rate Conversion) pattern.
2. The Related Arts
Today's vigorous development of science and technology brings constantly innovated information products to suit various needs of the public. Displays of the early time are most CRT (Cathode Ray Tube) displays, which are bulky and consume a large amount of electrical power and may generate radiation that is hazard to body health for users who use the displays for a long time. Thus, for the displays that are currently available in the market, liquid crystal displays (LCDs) are gradually taking the place of the CRT displays.
The liquid crystal displays have a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and is thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The operative principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and the liquid crystal molecules are controlled to change direction by application of electricity in order to refract out light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images.
Driving achieved through alternate-current driving is often adopted to drive the liquid crystal module of an LCD. Alternate-current driving is an essential characteristic of LCD liquid crystal module and regular LCD liquid crystal modules uses alternate-current signals to prevent formation of charge accumulation on upper and lower substrates of liquid crystal cell. A pixel is operated by alternately applying positive voltage and negative voltage (positive and negative being determined with reference to ITO voltage of color filter) to drive rotation of liquid crystal molecules, with “frame” as time unit.
FRC (Frame Rate Conversion) is a commonly used gray level expansion method. A grey level between two adjacent grey levels can be created through combined use of space and time. Such a technique is often applied to white tracking of LCD liquid crystal module for expanding the number of colors that can be shown by a liquid crystal panel so as to provide expanded flexibility of selection for color mixture operation.
Referring to FIGS. 1 and 2, the FRC grey level expansion method generally comprises two types, namely space domain FRC and time domain FRC. The space domain FRC is based on the fact that naked eyes of human beings cannot distinguish a single pixel and is operated by alternately setting adjacent grey levels on adjacent pixels so that the grey levels perceived by human eyes show an intermediate grey level. The time domain FRC is based on the fact that naked eyes of human beings cannot distinguish the image of a single frame (which is around 16.7 ms for 60 Hz) and is operated by alternately displaying adjacent grey levels on the same pixel so that the grey levels perceived by human eyes show an intermediate grey level.
The process of FRC is generally defined by a timing control chip (Tcon) of a driving circuit for liquid crystal module. To lower down the potential risks of deterioration of resolution in time domain FRC and reduction of frame rate in space domain FRC, the timing control chip generally adopts a combined process of both space domain FRC and time domain FRC, as illustrated in FIG. 3.
Generally, the alternate-current driving and FRC are both important measures for enhancing quality of liquid crystal panel and have both been widely used. However, in certain applications, they cause optical issues (such as the displayed image showing alternate occurrences of bright and dark strips). For example, in an alternate-current driving method that adopts dot inversion, if FRC is used in combination to display a 0.5-level image of a pure color, then data signals will pull down the voltage of COM terminal, leading to periodical cross-talking and also making the display image showing water ripples.
The cause of these phenomena is illustrated in FIG. 4, where (a) is the inversion state in simply displaying green screen; (b) is the corresponding FRC patterns; (c) shows the coupling directions of the data signal to Vcom, wherein negative voltage is downward and positive voltage is upward when inversion an FRC are applied simultaneously; and (d) shows the change of frames, where asymmetric situation of the Vcom coupling is not eliminated and water ripple is observed.
Generally, Vcom will be coupled to show very low voltage and the influence caused by its superposition on signal voltage is very minute. However, for low grey levels, the V-T curve has a small slope and the voltage difference between adjacent grey levels is great. Under this condition, the coupling of Vcom shows significance. Consequently, water ripple is often observed in low grey level images.